In order to produce electrically conductive connections between the individual conductor planes of multilayer printed circuit boards, methods are known for the electro-chemical metallization (through-plating) of previously drilled or punched holes. This type of through hole plating is normally used only in the case of epoxy resin glass laminates and is technically very complex, and hence expensive.
In the case of two-layer printed circuit boards consisting of base materials which are copper-laminated on both sides, it is known for the electrically conductive connection of conductor tracks on both sides to be produced by pressing an electrically conductive paste into previously drilled holes. The conductive paste is subsequently cured under the influence of relatively high temperatures. This method suffers from insufficient reliability of the through hole connecting.
DE-A-1,540,249, EP-A2-0,137,279, and FR-A-1,464,288 each disclose methods in which a further, relatively thin printed circuit board layer consisting of a carrier material layer with a ready-etched conductor pattern is applied onto a carrier board. The electrically conductive connection of the contact areas or connecting areas of the individual planes (layers) takes place by soldering, through hole connecting taking place only in the region of the leads of component parts. In consequence, the practical usefulness of the method is limited.
A similar method is disclosed in U.S. Pat. No. 3,052,823, the laminated-on second or further printed circuit layer having openings in order to obtain access to the contact areas of the conductor layer located underneath in each case. The through-connecting takes place by means of soldering in the edge regions of these openings, it being possible to connect contact points or connecting points which lie side-by-side directly, that is to say without the aid of connecting wires. This method provides sufficient reliability only in the case of hand soldering, so that large-scale industrial use could not be implemented.
DE-B1-1,923,199 discloses a method for producing a circuit board having at least two conductor layers which are separated from one another by insulating layers, in the case of which method the conductor layers are connected at points by means of contact spots, which are coated for example with solder, through openings in the insulating layers. The through-connecting takes place by pressing on the contact areas, using selective pressure, the outer layer being pressed onto the inner layer and being connected thereto. U.S. Pat. No. 4,319,708 also discloses a similar method. These methods are extremely complex and do not permit the production of high density multilayer circuit boards.
A multilayer is described in EP-A 0 146 241. Conductor layers consisting of a conductive resinous composition are produced on the opposite surfaces of at least two carrier materials and an interposed carrier material, . At determined points the carrier material is punched and all carrier materials subsequently are thermolaminated. By this a multilayered circuit board is produced without pads and external carrier material on both sides. The technique described in the aforementioned document, however, cannot be applied for metallic printed circuit.
Finally, DE-A1-3,152,603 (=EP-A1-0,067,231) discloses a method of the generic type in which an insulation coating which leaves the contact areas free is initially applied onto a carrier board having a first conductor layer and contact areas. A copper foil is laminated onto this insulation coating, via an adhesive coating, the adhesive coating and the copper foil having holes at the points at which the contact areas of the first conductor layer are located. Subsequently, the second conductor layer having connecting areas is produced from the copper foil by etching, the connecting areas being constructed as solder eyes which surround the holes in the adhesive coating. The connecting of the solder eyes with the contact areas takes place by the application of a conductive material, such as silver conductive paste or solder. In this method, the contact areas of the first conductor layer are separated from the solder eyes of the second conductor layer only by the thickness of the adhesive coating. Nevertheless, connection of the contact areas to the solder eyes by automatic soldering, for example by wave soldering, is not sufficiently reliable. DE-A1-2,716,545 also discloses a similar method.